1. Field of the Invention
This invention relates generally to thermal-inkjet (TIJ) pens in a printing machine; and more particularly to a system for sensing ink discharge to control pen-priming or document creation (or both) using such a printer.
2. Prior Art
TIJ pens sometimes are subject to nozzle failure. Such failure can be particularly problematical when it affects only a few of the nozzles in a pen, because operators of a printer do not generally think about inspecting for proper operation of every nozzle.
In general, failure of one or a few nozzles will affect only certain specific kinds of imprinted features. For example, in printing of alphabetic characters, such failure may degrade perhaps only some small elements of only certain letters--such as perhaps the serif on a letter "j", or one end of the crossbar on a "t".
These features are not conspicuous when a printing-machine operator merely glances at a page of text. They may be quite unacceptable, however, to a supervisor, quality-control manager, or customer.
Even if it were to occur to an operator to check operation of every nozzle in a newly installed pen, the necessary procedure--merely for checking the nozzles--would be obscure. It would have to be learned, and would be somewhat tedious.
Then if one more nozzles turned out to be nonfunctioning, or not functioning reliably, the operator of a printer heretofore would have little alternative but to print with an at least partially inoperative pen--or discard the pen and start again with a new one. This resolution of the matter would be undesirably expensive.
It would also be very wasteful, since virtually all new pens do in fact contain ink and in principle nearly all nozzles can in fact be started and made to flow reliably by suitable techniques. In many cases the operator may succeed in getting all the nozzles to work properly simply by operating the pen for a while, but the operator would have no way to determine in advance whether this effort would eventually succeed or, if so, how much time might be required to do so.
As a result, a large document may be printed, and perhaps copied, before significant printing defects are noticed. Fortunately this is rare, but its rarity tends to make it less likely to be noticed in time to prevent wasting time, paper (or other printing-medium stock), and money.
At least one prior printing machine does include a separate station into which a pen can be inserted for manually initiated priming. This arrangement is quite useful, but does require additional knowledge, time and care on the part of the operator--to remove the pen from its normal operating position, install it in the priming station, operate the priming apparatus, move the pen back into the normal position and try again. This system also typically requires iterating the procedure to some appropriate extent--and may call for some operator sophistication to decide what that extent is.
After a pen has been primed and ink can flow from each nozzle, usually ink flows reliably unless the system is out of service for protracted periods or is subjected to unduly harsh handling. In such cases unreliable operation occasionally recurs.
Later, however, as the ink supply nears exhaustion, once again ink flow becomes uncertain; and again automatic monitoring of the electrical actuating system does not readily reveal the moment when ink actually runs out. Consequently an operator may come back to a printer that has been directed to produce a long document to discover that some or many pages are blank.
Of course this type of printing failure is much easier to detect, for just a glance at the most recently ejected sheet reveals it. An operator who happens to notice that a printer is ejecting blank pages can stop the machine to replenish the ink.
Nevertheless such observation does require significant vigilance. The operator of a printer generally has other responsibilities, which can be handled more efficiently if not interrupted for monitoring a printer.
Just inside, or part of, each nozzle of a typical TIJ pen is a tiny thin-film resistive heater--controlled by actuating signals from a microprocessor through pen-drive circuitry, and positioned to heat and vaporize a very small volume of ink. Just ahead of this vaporized volume, an ink drop is expelled from the nozzle by abrupt expansion of the vapor.
Heretofore TIJ printing machines could automatically confirm passage of the actuating signals to the pen--and even, to some extent, could confirm operation of the thin-film resistor and other actuating elements within the pen that receive those signals and discharge the ink. For example, the DeskJet.RTM. and PaintJet XL300.RTM. printers produced by Hewlett Packard Company of Palo Alto, Calif., automatically test for open circuits and TIJ-nozzle actuating resistors; however, these printers cannot determine whether the nozzles are actually working--except by printing a test pattern for human visual observation, and for human discrimination between performances of the different nozzles.
Several mechanical phenomena sometimes prevent or partially inhibit ink flow, even when the actuating system is working. These phenomena include, but are not limited to:
exhaustion of ink, PA1 ink crusting, PA1 viscous plugs (i.e., increase in ink viscosity, generally due to exposure to air), PA1 open nozzle-actuating resistors, PA1 open or poor connectors between electronic modules, PA1 open or intermittent trailing cables on printers with scanning print heads, and PA1 malfunctioning pen-drive circuitry, PA1 (1) determining whether the at least one signal indicates inadequate discharge of ink from the pen and, if so, PA1 (2) directing a more-energetic priming impulse to the pen. PA1 the at least one signal indicates adequate discharge of ink from the pen, or PA1 a priming impulse of maximum suitable energy has been applied. PA1 ink resupply, or PA1 an operator's command to proceed despite inadequate ink discharge.
U.S. Pat. No. 4,922,270--issued May 1, 1990, to Cobbs, Haselby (one of the present inventors) and Osborne--teaches use of an ink-drop detector to synchronize operation of pens in a multipen printer. That document, however, suggests no other practical use for information from such a detector.
As can now be seen, important aspects of the technology which is used in the field of the invention are amenable to useful refinement.